RDMA/core: No need to protect kfree with spin lock and semaphore

While unregistering a client, only context removal should be protected
with lock. There is no need to protect a freeing of such context which is
already removed from the list.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>